1. Field of the Invention
The present invention relates to a thin film piezoelectric resonator which can be produced using micromechanical methods.
The resonant frequency of thin film piezoelectric resonators in the frequency range above 500 MHz is indirectly proportional to the layer thickness of the piezoelectric layer. The carrier membrane and the bottom and cover electrodes constitute an additional mass loading for the resonator which effects a reduction in the resonant frequency. The thickness fluctuations in all these layers determine the range of manufacturing tolerances in which the resonant frequency of a specimen resonator lies. Layer thickness fluctuations of 5% are typical for sputtering processes in microelectronics. A 1% tolerance can be achieved with a considerable outlay. Fluctuations occur both statistically from wafer to wafer and systematically between the middle of the wafer and the edge. The resonant frequencies of individual resonators must exhibit an absolute accuracy of 0.5% for filters in the GHz range.
A plurality of resonators must be connected in a ladder configuration, lattice configuration, or parallel configuration for highly selected filters. The individual resonators must be detuned specifically relative to one another in order to achieve the desired filter characteristic. It is preferable for reasons of cost to produce all the resonators of a filter from a piezoelectric layer of constant thickness; frequency tuning is performed by additive layers on the cover electrodes. An additional layer of different thickness must be produced for each resonant frequency which occurs. This requires in each case a deposition or an etching step, connected to a lithography step. In order to limit this outlay, it is customary to produce only filter topologies with the aid of which only two resonant frequencies are set.
The resonant frequency of thin film piezoelectric resonators can basically be trimmed by applying additional layers, as described above, but this necessitates expensive lithography. Material can be removed over the entire surface by laser trimming or ion-beam trimming, and this reduces the mass of the cover layer, although necessitating an expensive fabrication step at the end of the fabrication process. It is true that the resonant frequency can be shifted by connecting capacitors or applying a d.c. voltage, but the trimming range is comparatively narrow. The same holds for thermal trimming by heating up the resonator.
2. Summary of the Invention
The object of the invention is to provide a thin film piezoelectric resonator which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which can be set to a prescribed resonant frequency using simple means and with high accuracy. It is a further object to specify how a plurality of resonant frequencies can be set in a simple way.
With the above and other objects in view there is provided, in accordance with the invention, a thin film piezoelectric resonator, comprising:
a lower electrode layer and an upper electrode layer;
a piezoelectric layer between the lower electrode layer and the upper electrode layer;
an additional layer disposed on the upper electrode layer, the additional layer having a structure setting a prescribed resonant frequency of the piezoelectric resonator.
In accordance with an added feature of the invention, the additional layer has holes defining the structure, and a spacing between each one of the holes and a respectively closest hole is smaller than a wavelength of an operating wavelength of the resonator.
In accordance with an alternative feature of the invention, the structure in the additional layer is defined by islands. Similarly to above, a spacing between each one of the islands and a respectively closest island is smaller than the resonator operating wavelength.
In accordance with another feature of the invention, the structure is defined in such irregular distribution and dimensioning that diffraction phenomena are avoided.
In accordance with a further feature of the invention, the piezoelectric layer is formed of AlN, ZnO, or PZT ceramic.
In accordance with a concomitant feature of the invention, a carrier film of polysilicon is disposed below the lower. electrode layer, the piezoelectric layer, and the upper electrode layer, and wherein a cavity is formed on a side of the carrier film averted from the lower electrode layer.
With the above and other objects in view there is also provided, in accordance with the invention, a piezoelectric resonator assembly, comprising a plurality of thin film piezoelectric resonators as outlined above. These thin film resonators are formed on a common chip and they are set to at least three different resonant frequencies.
In summary, the thin film piezoelectric resonator of the invention has, in the layer of the cover electrode, or in an additional layer specifically applied therefor, holes, preferably produced lithographically, or similar structures. The structures have a mean spacing from one another which is smaller than the acoustic wavelength provided during operation of the component. These structures are preferably distributed with a uniformity that is sufficient to effect a uniform change in the mass of the layer per area (area density), thus producing a specific setting of the resonant frequency/ frequencies. On the other hand, the structures are preferably distributed so irregularly that diffraction effects are avoided.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a thin film piezoelectric resonator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.